Evolution of the circumstellar molecular envelope. I. A bima CO survey of evolved stars

This paper reports the results of a small imaging survey of eight evolved stars including two AGB stars (IRC + 10216 and Mira), five proto-planetary nebula (PPN) candidates (AFGL 2688, IRAS 22272+5435, HD 161796, 89 Her, and HD 179821), and a planetary nebula (PN, NGC 7027). We present high-resolution (CO)-C-12 J = 1 -> 0 maps of their full molecular envelopes made by combining BIMA Millimeter Array and NRAO 12 m telescope observations. For the PPNe and PN, the neutral molecular envelopes are compared with images taken at optical, near-IR, and mid-IR wavelengths. Drawing from the literature, we augmented our BIMA survey sample to 38 well-studied sources with CO emission maps. We classified this sample of sources based on the kinematics and morphologies of the CO emission into three types: spherical/elliptical/shell sources, disk sources, and structured outflow sources. Confirming previous studies, we find strong evidence for the photodissociation of the molecular envelope as an object evolves from the AGB to PN stages. While the spherical AGB stars follow theoretical expectations for mass-loss rate versus envelope size, the post-AGB structured outflow sources have significantly higher mass-loss rates than expected probably because of their recent superwinds. We find evidence that the structured outflows are clearly younger than the AGB wind. The disk sources have little correlation between mass-loss rate and envelope size because their properties are determined more by the properties of the central stars and disk evolution than by the mass-loss rate history that shapes the spherical and structured-outflow sources.